The latest deadline issued by Taleban rebels threatening to kill a group of 23 South Korean hostages in Afghanistan has passed with their demands unmet.

The rebels told Afghan authorities to trade Taleban prisoners for the hostages by 1900 local time (1430 GMT).

But officials say the Taleban rebels issued a new ultimatum soon after that deadline expired.

Intense negotiations have been taking place between the two sides since the Koreans were abducted on Thursday.

The militants have extended their ultimatum on the fate of the South Koreans at least three times.

Meanwhile, South Korea has added Afghanistan to a list of countries its citizens are banned from travelling to.

Any South Korean making an unauthorised journey to a banned country can be jailed for up to one year or fined 3m won ($3,200).

The foreign ministry has urged South Koreans in Afghanistan - believed to number about 200 - to consider leaving.

South Korea also has about 200 peacekeeping troops in the country, which Seoul had already been planning to withdraw by the end of the year.

Delicate diplomacy

The South Koreans were seized from a bus travelling from the city of Kandahar to the Afghan capital, Kabul.

They are reported to be Christians on an evangelical and aid mission. At least 15 are said to be women.

The seizure is the largest-scale abduction of foreigners since the fall of the Taleban regime in 2001.

The BBC's Charles Haviland, in Kabul says delicate diplomacy, not military muscle, is at the forefront of efforts to get the South Koreans out safely.

An eight-strong South Korean delegation, including a presidential envoy, is in Kabul to meet Afghan President Hamid Karzai and negotiate for the hostages' release.

Afghan elders have also been mediating between the militants and government negotiators in central Ghazni province, where the group was taken.

The hostages are reported to be in good health, but the rebels have said they will kill them if there is any attempt to free them by force or if the government fails to release a number of Taleban prisoners soon.

The group has also called for South Korean troops to leave the country.

German hostages

On Sunday, police in Wardak province said they had found the body of one of two German hostages kidnapped last Wednesday.

The Germans, whose identity has not been revealed, were seized with a number of Afghans in Wardak, where they had been working on a dam project.

A Taleban spokesman said both men were killed on Saturday because Germany refused demands to withdraw its 3,000-strong force from the country.

But Berlin said it believed one hostage was still alive and the other died of either a heart attack or of stress.

Chancellor Angela Merkel on Sunday evening said Germany would not give in to Taleban "blackmail" to withdraw its troops.